A 4-year-old child develops a runny nose and cough. After the cough persists for 2 weeks she exhibits paroxysms of coughing so severe she becomes cyanotic. On physical examination, her temperature is 37.4deg C. Her mouth and pharynx reveal no erythema or swelling. On auscultation of the chest, her lungs show crackles bilaterally. She has spasmodic coughing, with a series of coughs on a single breath, bringing up mucus plugs, followed by labored inspiration. The pathogenesis of her disease most likely results from disabling of which of the following?